The present invention relates to a unique form of expandable metal foil and to expanded metal nets made therefrom. The invention also relates to methods and apparatus for producing the said products, and to uses thereof, particularly in the extinguishing of fires and the prevention of explosions.
Surface fires, such as grassland and forest fires, as well as fires on the surface of water and on the surface of fuels in fuel tanks, are a continuing threat to life and property throughout the world. Over the years, numerous methods for combating such fires have been developed. The use of water, foams, chemicals and other quenching materials are well known.
It is also known to use blankets, mats, nets and other sheet-like materials to smother surface fires. However, these are heavy, bulky materials, and their use in widespread surface fires extending over thousands of acres of land or water, are subject to obvious limitations. Firefighting methods today are still limited to the steps of containing the fire as much as possible until it burns out or until changing weather conditions no longer support the burning. There is a need for a more efficient, inexpensive means for extinguishing fires which extend over wide surface areas.
There is also a need for more effective ways of preventing explosions in containers for fuels or other explosive substances. Containers such as fuel depots, liquid petroleum gas tanks, airplanes, ships, transport tankers, pipelines, and the like, are at risk from explosion caused by overheating, static electricity build up, mechanical impacts, etc. In addition to precautionary measures such as avoiding the above causes, a more recent approach to the problem has involved placing in the container a quantity of filling material in the form of a honeycomb shaped metal netxe2x80x94either in sheets or crumpled into balls. The theory of such approach is that the metal net promotes heat conduction and avoids static electricity build up, and thus reduces the risk of explosion. Although the approach has merit, there is nevertheless a substantial need for improvement, mainly because of deficiencies in the physical characteristics of the metal nets and balls, and also because of inefficiencies in the methods and apparatuses for producing such materials.
It is an object of the present invention to provide a product which is substantially more effective than known products, not only in the extinguishing of surface fires but also in the prevention of explosions in fuel tanks and the like.
It is a further object of the invention to provide a fire extinguishing product which can be transported to the site of a surface fire in compact, semi-manufactured form and then stretched to its fully manufactured form as it is applied to the surface of the fire over an extended area.
It is another object of the invention to provide a product for filling into containers for fuel and other explosive materials to provide a highly superior anti-explosive protection.
It is a still further object of the invention to provide unique methods and apparatus for production of the said new product.
Other objects and advantages will become apparent as the specification proceeds.
This invention is based on the development of a new form of an expandable slit metal foil which may be stretched into a three-dimensional metal net having unique properties. The expanded metal net is useful in extinguishing surface fires and also in the prevention of explosions in fuel containers and the like. It is also useful for other purposes, which will be explained hereinafter.
In one of its forms, the product of the invention is an expandable metal product comprising a continuous sheet of metal foil having discontinuous slits in spaced apart lines parallel to each other but transverse to the longitudinal dimension of said sheet. When said continuous sheet is stretched longitudinally, it is transformed into a three-dimensional metal net, and when said net is laid over a surface fire the fire is smothered and thus extinguished.
The fire extinguishing capability of the metal net is based on the phenomenon that flame at the surface of a burning material cannot pass upwardly through the pores or eyes of the metal net. In a normal fire, the heat of the burning causes material at the surface of the fuel to vaporize and mix with the oxygen in the atmosphere above it to produce a flammable mixture. If the metal net of the present sent invention is interposed between the surface of the burning material and the atmosphere, the heat conductivity of the metal net reduces the heat of the fire and thus reduces the amount of vapor being produced. The net also prevents the flame at the surface of the burning material from reaching the flammable mixture of vapor and atmosphere above the fire, and for these two reasons the conditions for continued burning are removed and the fire is extinguished.
The expandable metal product of the present invention provides a significant advantage in the fighting of fires covering a large surface area. In producing the expandable product, rolls of continuous metal foil are passed through banks of slitting knives to provide lines of discontinuous slits which are parallel to each other but transverse to the longitudinal dimension of the continuous sheet. The slitted sheet is then, in the same process, and without stretching, collected on a roll, ready for transportation to the site of a fire. In their unstretched form, the rolls are very compact, and large numbers of them can be transported by aircraft or other means to the location of a fire. At the fire, the metal foil is unrolled and stretched as it is applied to the surface of the fire. The stretching of the expandable product increases the surface area by approximately a tenfold factor. For example, if a roll of this material in its unstretched form is 44 cm wide and 500 m long, it will cover 220 square meters in its unstretched form, but this will be increased to 2,000 square meters in its stretched form. It will thus be seen that a substantial advantage is gained in terms of transporting the raw material in compact lightweight form and then transforming it by stretching to cover large areas of burning surface at the site of the fire.
In a specific embodiment of the invention, the rolls of slitted foil in the unstretched form can be carried in airplanes or helicopters over a burning area, and weights can be applied to the ends of the sheets, such that, as the weights fall toward the burning area, the foil unrolls and is stretched as it unrolls, thus covering the greatly expanded area of the stretched metal net.
It is a feature of the invention that, in the manufacture of the expandable metal foil, the transverse slit lines are made to extend to the longitudinal edges of the foil sheets, thus eliminating unslit longitudinal margins which might resist longitudinal stretching of the slit sheet when subjected to longitudinal tension. This feature enables the rolls of expandable metal foil to be stretched into metal nets as they are unrolled at the sites of fires, thus providing the very substantial gain in area of coverage, as described above.
In another of its forms, the metal net of the present invention is formed into small ellipsoid shapes which, by themselves or in combination with large sheets of expanded metal net, are useful not only for extinguishing surface fires but also for filling containers of fuel to prevent explosions therein. If the ellipsoids are to be used on the surface of water or other liquid, they are provided with floatable cores. In the practice of one embodiment of the invention, such ellipsoids are placed on the surface of the liquid fuel in a fuel tank and provide a floating surface layer on said liquid. The ellipsoid shape enables the units to nestle together on the surface, eliminating vacant spaces between them, thus providing a continuous surface cover with no gaps through which flame from the liquid can upwardly escape. In another embodiment, the ellipsoids are used to completely fill large or small containers of fuel, for the purpose of preventing explosion of the fuel; and in this arrangement also, the ability of the ellipsoids to nestle together provides a superior gap-free configuration. In this respect, the ellipsoidal units of the present invention are superior to metal nets which are crunched into the shape of spheres, since a layer of floating spheres inevitably leaves gaps or spaces between the spheres, through which flame or heat from the liquid fuel can escape upwardly.
In the practice of another embodiment of the invention, the above-described ellipsoids with floatable cores are distributed over a fire burning on the surface of water, and then sheets of the expanded metal net of the present invention are laid in place on top of the floatable ellipsoids, thus preventing the sheets of expanded metal net from sinking below the surface. In the practice of a further embodiment, the above-described ellipsoids are distributed in large numbers on the surface of land fires, and the ability of the ellipsoids to nestle together with each other provides a continuous layer of metal net for smothering the fires, similar to the manner in which the sheets of expanded metal net operate.
The present invention also relates to apparatus for producing an expandable metal product comprising a pair of opposing rotatable cylinders, means for rotating said cylinders at substantially the same speed, and means for passing a continuous sheet of metal foil between said cylinders, the first of said cylinders having spaced apart discontinuous knives attached to its outer surface in lines transverse to the longitudinal dimension of said continuous metal sheet, and the second of said cylinders having corresponding base members cooperating with said knives to produce lines of discontinuous slits in said continuous sheet of metal foil. In a variation of said apparatus, the slitting knives are replaced by spaced punches for the production of perforated sheets of metal foil.
A further embodiment of the invention relates to apparatus for forming sections of expanded metal foil into ellipsoidal shapes and for inserting floatable balls or other materials on the interior of said ellipsoids during the manufacture thereof.